arXiv subjects

Shigeru Wakita

Publications and source records attributed to Shigeru Wakita.

At least 19 recordsLinked to original sources

arXiv2026

Formation of Ryugu's parent planetesimal beyond the CO$_{2}$ snow line from small pebbles: insights from thermal evolution modeling

Astronomical observations of planet-forming circumstellar disks indicate that planetesimals form from 0.1-mm- to 1-cm-sized dust aggregates, commonly called ``pebbles.'' When such pebbles accrete beyond the water snow line, the resulting icy planetesimals undergo water--rock differentiation and develop porous pebble-pile cores whose voids are saturated with liquid water. Circulation of this water enhances heat transport in the core, suppressing the temperature rise caused by the decay of radionuclides. In this study, we constrain the accretion age and constituent pebble size of Ryugu's parent planetesimal by modeling the thermal evolution of icy planetesimals and comparing the results with the precipitation ages and temperatures of aqueously formed minerals identified in samples returned from asteroid Ryugu. Our numerical results suggest that Ryugu's parent planetesimal accreted within 2.0 Myr of the formation of calcium--aluminum-rich inclusions. The inferred early accretion age supports the hypothesis that Ryugu's parent planetesimal formed earlier than most chondrule-bearing carbonaceous chondrite parent planetesimals, potentially explaining the absence of chondrules in Ryugu samples. We also found that the core of the parent planetesimal was composed of pebbles no larger than a few millimeters. Such small pebble sizes are consistent with theoretical predictions for planetesimals formed beyond the CO$_{2}$ snow line.

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arXiv2026

Effect of lunar rotation on antipodal ejecta deposition

The Moon's crust exhibits strong magnetic anomalies that likely record ancient magnetic fields produced by a core dynamo. Lunar materials have low magnetic susceptibility, but impactor material with high susceptibility may record the Moon's ancient dynamo. Some magnetic anomalies are antipodal to lunar basins and it is hypothesized that the impact ejecta accumulates at the basin antipode and records the ancient lunar magnetic field. Previous impact simulations, which ignored the rotation of the Moon, support this hypothesis. Here, we reevaluate the deposition of antipodal ejecta by considering the rotation of the Moon. Assuming that the rotational period of the Moon is the same as the current day, we find that the rotation of the Moon does not strongly affect the deposition of the antipodal ejecta if impacts occur near the poles. In contrast, rotation has a considerable effect on antipodal deposits for impacts occurring near the equator. An eastward impact at a 45 degree incidence angle results in almost no antipodal ejecta, however, an eastward impact with a 30 degree incidence angle accumulates more antipodal ejecta than the non-rotating case. It is hard to determine the impactor's direction and location, for basins without an ellipsoidal shape and/or an asymmetric ejecta blanket. Nevertheless, our work demonstrates that care must be taken in interpreting the magnetic anomalies antipodal to lunar basins and that the effect of rotation should not be ignored.

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arXiv2026

Chondrite Parent Bodies as Escaped Satellites of Proto-Planetary Embryos

Chondrites are composed of formerly partially molten material, known as chondrules, surrounded by fine-grained matrix. They date from the earliest times in Solar System history. However, their role in the formation of the planets is uncertain because, in part, it is not clear how they were produced. Here, we show a robust pathway for forming meteorite-producing asteroids that contain chondrules through embryo-embryo collisions during the late stages of terrestrial planet formation. Melted material from these impacts cool into chondrules and mix with unmelted material in embryo-centric disks that formed from the ejecta. This material accretes into numerous asteroid-sized satellites. These objects are later ejected onto heliocentric orbits because of gravitational encounters with other embryos, thereby becoming the parent bodies of chondrites. This mechanism provides a pathway to form chondrites in Solar System history at times commensurate with measured chondrule ages, while explaining many of their physical properties.

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arXiv2026

Numerical Simulations of Hypervelocity Micrometeoroid Impacts: Rocky Impactors onto Icy Targets and the Role of Porosity

In the outer Solar System, for example in the Saturnian system, a planet's strong gravity attracts micrometeoroids and generates hypervelocity impacts on bodies such as rings and satellites. Micrometeoroids are seemingly non-icy, whereas the targets are typically icy, and both the impactor and the target may span a wide range of porosities. In this study, we perform three-dimensional iSALE simulations of hypervelocity impacts of rocky impactors onto icy targets, varying the impact angle and the porosities of the impactor and target ($ϕ_{\rm imp}$ and $ϕ_{\rm tar}$). We consider two end-member porosities (0% and 90%) for oblique ($45^\circ$) impacts. At an impact velocity of 30 km/s, characteristic of Saturn's rings, we find that the morphology of early-stage crater formation varies significantly with porosity, transitioning from deep-penetration, narrow-channel cavities ($ϕ_{\rm imp}=0$, $ϕ_{\rm tar}=90%$) to very shallow craters driven by near-surface vapor blowoff ($ϕ_{\rm imp}=90%$, $ϕ_{\rm tar}=0%$), with intermediate, more hemispherical cavity shapes when the porosities are comparable. Here, we focus on the thermodynamic fate of the impactor, which represents the exogenic material responsible for modifying the target surface. The impactor material is strongly heated and is efficiently vaporized regardless of the porosities of the impactor and target. However, the peak pressure and peak temperature experienced by the impactor vary by nearly an order of magnitude. These results imply that hypervelocity impacts occurring, for example, in Saturn's rings efficiently vaporize exogenic non-icy impactors upon impact, while the subsequent thermodynamic pathways $-$ such as condensation and chemical evolution $-$ may differ depending on the thermodynamic conditions. Our results are expected to be applicable to a variety of planetary systems.

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arXiv2026

A youthful Titan implied by improved impact simulations

The small number of impact craters found on Titan suggests that its surface is relatively young. Previous work estimated its surface age to be between 200 and 1000 Myr. This estimate, however, is based on crater scaling laws for water and sand, which are not representative of the composition of Titan's icy surface. Titan's surface is likely composed of water ice, methane clathrates, or a combination of both. Here, we perform impact simulations for impactors of various sizes that strike an icy target with a 0-15 km thick methane clathrate cap layer. We derive new crater scaling laws based on our numerical results, and find that Titan's surface age is 300-340 Myr, assuming heliocentric impactors and surface clathrates. This age, which represents the crater retention age, indicates a relatively youthful surface, suggesting that active endogenic and/or exogenic processes have recently reshaped Titan's surface.

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arXiv2025

Impacts into Titan's methane-clathrate crust as a source of atmospheric methane

Titan is the only icy satellite in the solar system with a dense atmosphere. This atmosphere is composed primarily of nitrogen with a few percent methane, which supports an active, methane-based hydrological cycle on Titan. The presence of methane, however, is intriguing, as its lifetime is likely much shorter than the age of the solar system due to its irreversible destruction by UV photolysis. To explain Titan's current atmospheric methane abundance, it is hypothesised that a replenishment mechanism is needed. One such mechanism may be crater forming impacts; a methane-clathrate layer potentially covering the surface of Titan may act as a reservoir that releases methane when disrupted by impacts. Here, we perform impact simulations into methane-clathrate layers to investigate the amount of methane released via impacts. Our simulations show that the amount of methane released into the atmosphere depends on both the impactor size and the methane-clathrate layer thickness. A single 20-km-diameter impactor releases up to 1% of Titan's current atmospheric methane mass; the effect of impact obliquity and surface porosity may further increase the released mass by a factor of 2-3. The release rate from impacts is lower than the net loss rate by photolysis, but the released methane mass via impacts can enhance the lifetime of methane in Titan's atmosphere by up to 3%. Menrva-sized (> 400 km diameter) crater-forming impacts directly liberate of $\sim$15% Titan's current atmospheric methane. The direct heating of the atmosphere by the impactor might contribute to additional crustal heating and methane release.

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arXiv2024

Chondrule survivability in the solar nebula

The lifetime of mm size dust grains, such as chondrules, in the nominal solar nebula model is limited to $\sim 10^{5}$ yr due to an inward drift driven by gas drag. However, isotopic and petrological studies on primitive meteorites indicate a discrepancy of $\gtrsim 10^{6}$ yr between the formation time of chondrules and that of chondritic parent bodies. Therefore chondrules should survive for $\gtrsim 10^{6}$ yr in the solar nebula against the inward drift without subsequent growth (i.e., planetesimal formation). Here we investigate the conditions of the solar nebula that are suitable for the long lifetime of chondrule-sized dust particles. We take the turbulent strength, the radial pressure gradient force, and the disk metallicity of the solar nebula as free parameters. For 1 mm-radius-chondrules to survive and keep their size for $\gtrsim 10^{6}$ yr, the suitable condition is a weak turbulence ($α\sim 10^{-6}$), a flat radial profile ($η\lesssim 10^{-3}$), and a high metallicity ($Z\sim 0.1$). This condition is qualitatively consistent with the characteristics of protoplanetary disks suggested by recent observations. We eventually propose that planetesimal formation may be induced by the disk evolution, e.g., the inside-out dispersal of the gas component due to the disk wind.

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arXiv2023

Survivability of Amorphous Ice in Comets Depends on the Latent Heat of Crystallization of Impure Water Ice

Comets would have amorphous ice rather than crystalline one at the epoch of their accretion. Cometary ice contains some impurities that govern the latent heat of ice crystallization, $L_{\rm cry}$. However, it is still controversial whether the crystallization process is exothermic or endothermic. In this study, we perform one-dimensional simulations of the thermal evolution of km-sized comets and investigate the effect of the latent heat. We find that the depth where amorphous ice can survive significantly depends on the latent heat of ice crystallization. Assuming the cometary radius of 2 km, the depth of the amorphous ice mantle is approximately 100 m when the latent heat is positive (i.e., the exothermic case with $L_{\rm cry} = + 9 \times 10^{4}$ J/kg). In contrast, when we consider the impure ice representing the endothermic case with $L_{\rm cry} = - 9 \times 10^{4}$ J/kg, the depth of the amorphous ice mantle could exceed 1 km. Although our numerical results indicate that these depths depend on the size and the accretion age of comets, the depth in a comet with the negative latent heat is a few to several times larger than the positive case for a given comet size. This work suggests that the spatial distribution of the ice crystallinity in a comet nucleus depends on the latent heat, which can be different from the previous estimates assuming pure water ice.

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arXiv2023

Modeling the formation of Selk impact crater on Titan: Implications for Dragonfly

Selk crater is an $\sim$ 80 km diameter impact crater on the Saturnian icy satellite, Titan. Melt pools associated with impact craters like Selk provide environments where liquid water and organics can mix and produce biomolecules like amino acids. It is partly for this reason that the Selk region has been selected as the area that NASA's Dragonfly mission will explore and address one of its primary goals: to search for biological signatures on Titan. Here we simulate Selk-sized impact craters on Titan to better understand the formation of Selk and its melt pool. We consider several structures for the icy target material by changing the thickness of the methane clathrate layer, which has a substantial effect on the target thermal structure and crater formation. Our numerical results show that a 4 km-diameter-impactor produces a Selk-sized crater when 5-15 km thick methane clathrate layers are considered. We confirm the production of melt pools in these cases and find that the melt volumes are similar regardless of methane clathrate layer thickness. The distribution of the melted material, however, is sensitive to the thickness of the methane clathrate layer. The melt pool appears as a torus-like shape with a few km depth in the case of 10-15 km thick methane clathrate layer, and as a shallower layer in the case of a 5 km thick clathrate layer. Melt pools of this thickness may take tens of thousands of years to freeze, allowing more time for complex organics to form.

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arXiv2022

Effect of impact velocity and angle on deformational heating and post-impact temperature

The record of impact induced shock-heating in meteorites is an important key for understanding the collisional history of the solar system. Material strength is important for impact heating, but the effect of impact angle and impact velocity on shear heating remains poorly understood. Here, we report three-dimensional oblique impact simulations, which confirm the enhanced heating due to material strength and explore the effects of impact angle and impact velocity. We find that oblique impacts with an impact angle that is steeper than 45 degree produce a similar amount of heated mass as vertical impacts. On the other hand, grazing impacts produce less heated mass and smaller heated regions compared to impacts at steeper angles. We derive an empirical formula of the heated mass, as a function of the impact angle and velocity. This formula can be used to estimate the impact conditions (velocity and angle) that had occurred and caused Ar loss in the meteoritic parent bodies. Furthermore, our results indicate that grazing impacts at higher impact velocities could generate a similar amount of heated material as vertical impacts at lower velocities. As the heated material produced by grazing impacts has experienced lower pressure than the material heated by vertical impacts, our results imply that grazing impacts may produce weakly shock-heated meteorites.

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arXiv2022

Methane-saturated layers limit the observability of impact craters on Titan

As the only icy satellite with a thick atmosphere and liquids on its surface, Titan represents a unique end-member to study the impact cratering process. Unlike craters on other Saturnian satellites, Titan's craters are preferentially located in high-elevation regions near the equator. This led to the hypothesis that the presence of liquid methane in Titan's lowlands affects crater morphology, making them difficult to identify. This is because surfaces covered by weak fluid-saturated sediment limit the topographic expression of impact craters, as sediment moves into the crater cavity shortly after formation. Here we simulate crater-forming impacts on Titan's surface, exploring how a methane-saturated layer overlying a methane-clathrate layer affects crater formation. Our numerical results show that impacts form smaller craters in a methane-clathrate basement than a water-ice basement, due to the differences in strength. We find that the addition of a methane-saturated layer atop this basement reduces crater depths and influences crater morphology. The morphology of impact craters formed in a thin methane-saturated layer are similar to those in a "dry" target, but a thick saturated layer produces an impact structure with little to no topography. A thick methane-saturated layer (thicker than 40% of the impactor diameter) could explain the dearth of craters in the low-elevation regions on Titan.

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arXiv2021

Jetting during oblique impacts of spherical impactors

During the early stages of an impact a small amount material may be jetted and ejected at speeds exceeding the impact velocity. Jetting is an important process for producing melt during relatively low velocity impacts. How impact angle affects the jetting process has yet to be fully understood. Here, we simulate jetting during oblique impacts using the iSALE shock physics code. Assuming both the target and impactor have the same composition (dunite), we examine the jetted material which exceeds the impact velocity. Our results show that oblique impacts always produce more jetted ejecta than vertical impacts, except for grazing impacts with impact angles $< 15^{\circ}$. A 45$^{\circ}$ impact with an impact velocity of 3 km/s produces jetted material equal to $\sim$ 7 \% of the impactor mass. This is 6 times the jetted mass produced by a vertical impact with similar impact conditions. We also find that the origin of jetted ejecta depends on impact angle; for impact angles less than 45$^{\circ}$, most of the jet is composed of impactor material, while at higher impact angles the jet is dominated by target material. Our findings are consistent with previous experimental work. In all cases, jetted materials are preferentially distributed downrange of the impactor.

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arXiv2019

Enhancement of impact heating in pressure-strengthened rocks in oblique impacts

Shock-induced metamorphism in meteorites informs us about the collisional environment and history of our solar system. Recently the importance of material strength in impact heating was reported from head-on impact simulations. Here, we perform three-dimensional oblique impact simulations, and confirm the additional heating due to material strength for oblique impacts. Despite a large difference in the peak pressure at the impact point at a given impact velocity, we find that the heated mass for an oblique impact is nearly the same as that for a head-on impact. Thus, our results differ from the previous finding that the heated mass decreases as the impact becomes more oblique, and show that the additional shear heating is more effective for oblique impacts than for head-on impacts. This also indicates that material ejected during oblique impact tends to experience lower shock pressures but higher temperatures.

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arXiv2019

Aggregate Growth and Internal structures of Chondrite Parent Bodies Forming from Dense Clumps

Major components of chondrites are chondrules and matrix. Measurements of the volatile abundance in Semarkona chondrules suggest that chondrules formed in a dense clump that had a higher solid density than the gas density in the solar nebula. We investigate collisions between chondrules and matrix in the surface region of dense clumps using fluffy aggregate growth models. Our simulations show that the collisional growth of aggregates composed of chondrules and matrix takes place in the clumps well before they experience gravitational collapse. The internal structure of chondrite parent bodies (CPBs) can be thereby determined by aggregate growth. We find that the aggregate growth generates two scales within CPBs. The first scale is involved with the small scale distribution of chondrules and determined by the early growth stage, where chondrules accrete aggregates composed of matrix grains. This accretion can reproduce the thickness of the matrix layer around chondrules found in chondrites. The other scale is related to the large scale distribution of chondrules. Its properties (e.g., the abundance of chondrules and the overall size) depend on the gas motion within the clump, which is parameterized in this work. Our work thus suggests that the internal structure of CPBs may provide important clues about their formation conditions and mechanisms.

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arXiv2019

The Properties of Planetesimal Collisions under Jupiter's Perturbation and the Application to Chondrule Formation via Impact Jetting

Understanding chondrule formation provides invaluable clues about the origin of the solar system. Recent studies suggest that planetesimal collisions and the resulting impact melts are promising for forming chondrules. Given that the dynamics of planetesimals is a key in impact-based chondrule formation scenarios, we here perform direct $N$-body simulations to examine how the presence of Jupiter affects the properties of chondrule-forming collisions. Our results show that the absence/presence of Jupiter considerably changes the properties of high velocity collisions whose impact velocities are higher than 2.5 km s$^{-1}$; high velocity collisions occur due to impacts between protoplanets and planetesimals for the case without Jupiter; for the case with Jupiter, eccentricities of planetesimals are pumped up by the secular and resonant perturbations from Jupiter. We also categorize the resulting planetesimal collisions and find that most of high velocity collisions are classified as grazing ones for both cases. To examine the effect of Jupiter on chondrule formation directly, we adopt the impact jetting scenario and compute the resulting abundance of chondrules. Our results show that for the case without Jupiter, chondrule formation proceeds in the inside-out manner, following the growth of protoplanets. If Jupiter is present, the location and timing of chondrule formation are determined by Jupiter's eccentricity, which is treated as a free parameter in our simulations. Thus, the existence of Jupiter is the key parameter for specifying when and where chondrule formation occurs for impact-based scenarios.

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arXiv2019

Fates of hydrous materials during planetesimal collisions

Hydrous minerals are found on the surfaces of asteroids, but their origin is not clear. If their origin is endogenic, the hydrous minerals that were formed in the inner part of a planetesimal (or parent body) should come out on to the surface without dehydration. If their origin is exogenic, the source of hydrous minerals accreting onto asteroids is needed. Collisions in the asteroid belt would be related to both origins because collisions excavate the surface and eject the materials. However, the fate of hydrous minerals in large planetesimals during the collisional process has not been well investigated. Here, we explore planetesimal collisions by using the iSALE-2D code, and investigate the effect of an impact for the target planetesimal containing hydrous minerals. Our numerical results for the fiducial case (5 km/s of the impact velocity) show that hydrous minerals are slightly heated during the collisions. This moderate heating indicates that they can avoid the dehydration reaction and keep their original composition. Some hydrous minerals have larger velocity than the escape velocity of the collision system. This means that hydrous minerals can escape from the planetesimal and support the theory of exogenic origin for the hydrous minerals on asteroids. Meanwhile, the velocity of other hydrous minerals is smaller than the escape velocity of the system. This also indicates the possibility of an endogenic origin for the hydrous minerals on asteroids. Our results suggest that hydrous minerals on asteroids can be provided by planetesimal collisions.

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arXiv2017

Chondrule Accretion with a Growing Protoplanet

Chondrules are primitive materials in the Solar System. They are formed in the first about 3 Myr of the Solar System's history. This timescale is longer than that of Mars formation, and it is conceivable that protoplanets, planetesimals and chondrules might have existed simultaneously in the solar nebula. Due to protoplanets perturbation on the planetesimal dynamics and chondrule accretion on them, all the formed chondrules are unlikely to be accreted by planetesimals. We investigate the amount of chondrules accreted by planetesimals in such a condition. We assume that a protoplanet is in oligarchic growth, and we perform analytical calculations of chondrule accretion both by a protoplanet and by planetesimals. Through the oligarchic growth stage, planetesimals accrete about half of the formed chondrules. The smallest planetesimals get the largest amount of the chondrules, compared with the amount accreted by more massive planetesimals. We perform a parameter study and find that this fraction is not largely changed for a wide range of parameter sets.

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arXiv2016

Planetesimal collisions as a chondrule forming event

Chondritic meteorites contain unique spherical materials named chondrules: sub-mm sized silicate grains once melted in a high temperature condition in the solar nebula. We numerically explore one of chondrule forming processes, planetesimal collisions. Previous studies found that impact jetting via protoplanet-planetesimal collisions make chondrules with an amount of 1 % of impactors' mass, when impact velocity exceeds 2.5 km s$^{-1}$. Based on the mineralogical data of chondrules, undifferentiated planetesimals would be more suitable for chondrule-forming collisions than potentially differentiated protoplanets. We examine planetesimal-planetesimal collisions using a shock physics code and find two things: one is that planetesimal-planetesimal collisions produce the nearly same amount of chondrules as protoplanet-planetesimal collisions ($\sim$ 1 %). The other is that the amount of produced chondrules becomes larger as the impact velocity increases when two planetesimals collide with each other. We also find that progenitors of chondrules can be ejected from deeper regions of large targets (planetesimals or protoplanets) than small impactors (planetesimals). The composition of targets is therefore important to fully account for the mineralogical data of currently sampled chondrules.

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